Mass flow meters operate on the principles of mechanics. Specifically, coriolis and/or gyroscopic forces are measured. These forces are proportional to the fluid mass flow.
Fluid mass flow meters based on a measurement of coriolis forces are disclosed in U.S. Pat. Nos. 4,187,721; 4,422,337 and 4,491,025. In those meters, the fluid moves through a "U"-shaped conduit.
A mass flow meter based on a measurement of gyroscopic forces is disclosed in U.S. Pat. No. 2,624,198. Fluid is passed through a conduit which is in the form of a coil. The ends of the coiled conduit are diametrically opposed and define an axis ("rotational axis") which is normal to an axis ("coil axis") of the coiled conduit. The coiled conduit is rotated or oscillated about the rotational axis. One end of the conduit is connected to a sensor. A flywheel is coaxial with the coiled conduit but is not coplanar. When fluid flows through the conduit, the conduit is rotated or oscillated about the rotational axis and the flywheel is not rotating; a couple is generated about a third axis. The third axis is perpendicular to the coil axis and rotational axis. This couple causes a deflection at the end of the coil connected to the sensor. The flywheel is rotated so to nullify the deflection. The flywheel's rotational speed is proportional to the fluid mass flow in the conduit.